1. Field of the Invention
This invention relates to novel compounds useful, for example, in optical switching, as well as a production method thereof.
2. Description of the Related Art
It has been known that some of dye derivatives, particularly, cyanine, porphyrin or squarylium dye derivatives form aggregates. The aggregate means a body in which several tens to several hundreds of molecules are regularly arranged and bonded loosely and behave optically as if they were a single super molecule. Particularly, the one shown in FIG. 5 having an absorption band FIG. 5B which is shifted toward a longer wavelength side and sharpened as compared with an absorption band FIG. 5A of a single molecule is referred to as J-aggregate.
It has been reported that the J-aggregate emits fluorescence with small Stokes shift, has extremely large interaction with light of a wavelength near the absorption peak and shows extremely rapid recovery from bleached absorption as a third-order nonlinear optical effect (M. Furuki, L. S. Pu, F. Sasaki, S. Kobayashi and T. Tani, Appl. Phys. Lett., 72, 21 (1998) 2648). Further, it has been confirmed that the optical response time of a thin film of squarylium dye aggregates formed on a solid substrate is 300 fs (1 fs=10xe2x88x9215 seconds) or less (Page 1 of Nikkan Kogyo Shimbun, dated Jul. 13, 1998). Further, a thin film of aggregates of a squarylium dye derivative having an ultrafast response characteristic of less than 100 fs which can be driven at an energy as low as 80 fJ/xcexcm2 has been attained (Page 5 of Nikkei Sangyo Shimbun, dated Jul. 8, 1999; M. Furuki, M. Tian, Y, Sato, L. S. Pu, H. Kawashima, S. Tatsuura and O. Wada, Appl. Phys. Lett., 78, 18(2001) 2634). With the feature described above, it is considered that a thin film of aggregates of a squarylium derivative can be used as an optical switch in optical information communication of terabit (1012 bit/s) order.
The present inventors have confirmed that the squarylium dye derivative forms aggregates on a solid substrate and they exhibit an ultrafast optical response characteristic of femtosecond order, for which patent applications were already filed (Japanese Published Unexamined Patent Application No. Hei 11-282034 and Japanese Published Unexamined Patent Application No. 2000-111967). However, since the ultrafast optical response of such a dye aggregate film can be obtained only for light of a wavelength near the absorption peak of the aggregates (780 nm), it is difficult at present to apply the film to optical switches that operate at a wavelength exceeding 1 xcexcm. Since the wavelength of the signal light in actual optical fiber communication networks is 1.3 xcexcm or 1.55 xcexcm, it is essential for the optical switch applicable to a communication system to use a dye having an intense absorption near 1.3 xcexcm or 1.55 xcexcm. On the other hand, there have so far been several kinds of near-infrared-absorbing dyes having maximum absorption wavelengths of 1 xcexcm or more (J. Fabian, H. Nakazumi and M. Matsuoka., Chem. Rev, 92 (1992) 1197).
However, the long-wavelength-absorbing dyes have a feature that the dye molecule includes a large xcfx80-conjugated system, and have the following problems:
1) The molar absorption coefficient of the dye is small.
2) The thermal stability is poor since the xcfx80-conjugated system is excessively long.
3) The solubility in organic solvents is poor, making it difficult to form a film.
Accordingly, the existent long-wavelength-absorbing dyes are difficult to apply to optical switches. In order to solve this subject, it is necessary to make improvements such as an increase of the solubility of the long-wavelength-absorbing dye, improvement of the thermal stability and sublimation property of the dye and control for the aggregation of the dye molecules in the film.
For the reasons described above, near-infrared-absorbing dyes having maximum absorption wavelengths of 1 xcexcm or more and having high thermal stability and good solubility have been demanded.
This invention provides novel compounds having maximum absorption wavelengths of over 1 xcexcm, and having high thermal stability and good solubility in organic solvents, as well as a production method thereof.
The present inventors have made earnest studies for attaining the foregoing goal and, as a result, have found dye molecules of a novel basic skeleton having an extremely effective acceptor structure at the center.
An aspect of the present invention provides a novel compound represented by a general formula (I): 
where R1 and R2 may be identical or different from each other and each represents a linear alkyl group or a branched alkyl group, respectively.
The compound shows maximum absorption near 1.1 xcexcm and the absorption wavelength thereof can be made longer without excessive extension of the conjugation system as in the existent long-wavelength-absorbing dyes. Further, it has high thermal stability and good solubility in organic solvents such as acetone and chloroform and, further, it is excellent also in view of sublimation property and film-formation property.
In the general formula (I), R1 and R2 may be identical or different from each other and each preferably represents a linear alkyl of 1 to 7 carbon atoms or a branched alkyl group of 3 to 7 carbon atoms, respectively, with isobutyl group being particularly preferred.
According to another aspect of the present invention, a compound of the general formula (I) is obtained by a production method including a step of reacting the aniline derivative represented by a general formula (II) with 4,5-dihydroxy-4-cyclopentene-1,2,3-trione shown by a general formula (III). 
In the general formula (II), R1 and R2 may be identical or different from each other and each represents a linear alkyl group or a branched alkyl group, respectively.